- EASY PREPARATION OF TRIPHENYLMETHYL CARBOXYLATES.
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A new method for triphenylmethylation of carboxylic acids and phenol is disclosed.
- Hashimoto,Hayashi,Noyori
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- Zinc chloride homogeneous catalysis in the tritylation of hydroxyl- and amide-bearing molecules
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A tritylation protocol based on the transfer of the triphenylmethylcarbenium ion from trityl acetate to substrates having hydroxyls, in the presence of catalytic amounts of ZnCl2, is described. The advantages of this method are broad scope, mild conditions, and easy handling. The comparison with the procedure based on the use of equimolar mixture of TrCl and ZnCl2 in the presence of TEA shows that comparable results are obtained. However, only this method allows reactions of secondary or benzylic alcohols such as oxidation or formation of symmetric ethers to be suppressed. Both procedures are successfully extended to simple and substituted amides. Irrespective of its low solubility in acetonitrile, even asparagine can be directly tritylated on its amide group.
- Maltese, Maurizio,Vergari, Maria Cecilia,Donzello, Maria Pia
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supporting information; experimental part
p. 483 - 487
(2011/03/18)
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- Stabilities of trityl-protected substrates: The wide mechanistic spectrum of trityl ester hydrolyses
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Ionization rates of para-substituted triphenylmethyl (trityl) acetates, benzoates, and para-nitrobenzoates have been determined in aqueous acetonitrile and aqueous acetone at 25 °C. Conventional and stoppedflow techniques have been used to evaluate rate constants ranging from 1.38 × 10-5 to 2.15 × 102S-1 by conductimetry and photospectrometry methods. The varying stabilities of the differently substituted tritylium ions account for a gradual change of reaction mechanism. Poorly stabilized carbocations are generated slowly by the ionization of their covalent precursors and trapped fast by water. Better stabilized carbocations are generated more rapidly and accumulate, so that ionization and trapping by water can be observed as separate steps in a single experiment. Finally, highly stabilized tritylium ions do not react with water, and only the rates of their formation could be measured. The ionization rate constants correlate linearly with Winstein's ionizing powers Y; the low slopes (0.17 + parameters is excellent for symmetrically substituted tritylium derivatives, deviations for unsymmetrically substituted systems are observed. The failing rate-equilibrium relationship between the rates of ionizations (log kion) and the stabilities of the carbocations in aqueous solution (pKR+) may be explained by the late " development of resonance between a p-amino group and the carbocationic center of the tritylium ion during the ionization process.
- Horn, Markus,Mayr, Herbert
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supporting information; experimental part
p. 7469 - 7477
(2010/09/11)
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- Nucleophilic acyl substitutions of anhydrides with protic nucleophiles catalyzed by amphoteric, oxomolybdenum species
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(Chemical Equation Presented) Among six different group VIb oxometallic species examined, dioxomolybdenum dichloride and oxomolybdenum tetrachloride were the most efficient catalysts to facilitate nucleophilic acyl substitution (NAS) of anhydrides with a myriad array of alcohols, amines, and thiols in high yields and high chemoselectivity. In contrast to the well-recognized redox chemical behaviors associated with oxomolybdenum(VI) species, the catalytic NAS was unprecedented and tolerates virtually all kinds of functional groups. By using benzoic anhydride as a mediator for in situ generation of an incipient mixed anhydride-MoO2Cl2 adduct with a given functional alkanoic acid, one can achieve oleate, dipeptide, diphenylmethyl, N-Fmoc-α-amino, pyruvic, and tert-butylthio ester, N-tert-butylamide, and trityl methacrylate syntheses with appropriate protic nucleophiles. The amphoteric character of the Mo=O unit in oxomolybdenum chlorides was found to be responsible for the catalytic NAS profile as supported by a control NAS reaction of using an authentic adduct-MoOCl2(O2-CBu t)2 between pivalic anhydride and MoO2Cl 2 as the catalyst.
- Chen, Chien-Tien,Kuo, Jen-Huang,Pawar, Vijay D.,Munot, Yogesh S.,Weng, Shieu-Shien,Ku, Cheng-Hsiu,Liu, Cheng-Yuan
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p. 1188 - 1197
(2007/10/03)
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- Bismuth(III) salts as convenient and efficient catalysts for the selective acetylation and benzoylation of alcohols and phenols
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Efficient acetylation and benzoylation of alcohols and phenols with acetic and benzoic anhydrides have been carried out under catalysis of bismuth(III) salts including BiCl3, Bi(TFA)3 and Bi(OTf)3. Selective acetylation and benzoylation of alcohols in the presence of phenols is an additional advantage of this procedure.
- Mohammadpoor-Baltork, Iraj,Aliyan, Hamid,Reza Khosropour, Ahmad
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p. 5851 - 5854
(2007/10/03)
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- ISOSELECTIVITY RELATION IN THE SOLVOLYSIS OF SUBSTITUTED TRITYL AND m-CHLOROTRITYL BENZOATES IN AQUEOUS ETHANOL
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The rate constants for the solvolysis of two series of trityl benzoates XC6H4(C6H5)2COC(O)C6H4Y in aqueous ethanol (80-100percent) were determined by potentiometric titration, and the activation and correlation parameters were calculated.In series (I) the effect of the substituent Y on the reaction rate in 100percent ethanol is due to entropic control.The isoselectivity temperature (Tis) lies in the region of 25 deg C.An inverse relation between the reactivity and selectivity (ρ, m) is observed when T > Tis, and this is regarded as the consequence of the predominance of solvation effects, which appear at the stage of the transition from intimate to loose ion pairs, over electronic effects.In the more reactive series (II) the effect of solvation decreases, the activation parameters level out, and the Tis value becomes less than the determined value.
- Karavan, V. S.,Eliseeva, E. E.
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p. 657 - 662
(2007/10/02)
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- TRIMETHYLSILYL TRIFLATE IN ORGANIC SYNTHESIS
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Trimethylsilyl triflate is a powerful silylating agent for organic compounds and acts as a catalyst which accelerates a variety of nucleophilic reactions in aprotic media.The reactions proceed via one-center, electrophilic coordination of the silyl group to hetero functional groups and exhibit unique selectivities.
- Noyori, R.,Murata, S.,Suzuki, M.
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p. 3899 - 3910
(2007/10/02)
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- A FACILE PROCEDURE FOR O-TRITYLATION
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The title operation is achieved via readily accessible organosilicon compounds.
- Murata, S.,Noyori, R.
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p. 2107 - 2108
(2007/10/02)
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